1. Field of the Invention
The present invention relates to zoom encoders and, more particularly, to a zoom encoder for use in a focal-length detecting apparatus of a zoom photographing optical system in a camera or the like.
2. Related Art And Prior Art Statement
As being known well, as a focal-length detecting apparatus of a zoom photographing optical system in a camera or the like, there are many arrangements in which an amount of angular movement and a direction of angular movement are detected by a zoom encoder of a cam ring which changes a focal length of the photographing optical system.
The zoom encoder is arranged such that, in order to detect a rotational position of the cam ring which executes zooming by rotation, a code plate of a few bits is provided, and a brush is in contact with the code plate to detect the rotational position of the cam ring. Detection of the rotational position due to this code plate has an advantage that the rotational position of the cam ring can be detected as bit information from the code plate, but has the following disadvantages: That is,
1) Manufacturing cost of the code plate is high;
2) Since the bit information is produced by contact of the brush with the code plate, conduction failure due to oxidization of a surface of the code plate and a surface of a brush occurs with the passage of time; and
3) Conduction failure due to insufficient pressure contact, wear or abrasion of the surface, or the like occurs with the passage of time.
Of these disadvantages, the fact that failure occurs with the passage of time, particularly like the above-described disadvantages 2) and 3) means that failure occurs after the focal-length detecting apparatus has been passed over hands of a user. This raises an extremely serious problem in view of the quality of products.
On the other hand, as means for solving the above-described disadvantages 2) and 3), it is considered to use an absolute-value encoder of non-contact type like an optical type using a photo-reflector or the like or a magnetic type using an MRE (Magneto-Resistive Element) or the like.
However, the use of such means is large or high in demerit in view of a needed space and cost thereof. The higher the resolution of a zoom encoder, the more the enlargement of the demerit. Means considered to be possible practically is limited at the most to one of 2-3 bits.
In view of the above, as disclosed in U.S. Pat. No. 5,122,822, means has been proposed in which an optical pulse generator is provided for generating pulses in response to rotation of a cam ring which executes zooming, a pulse output from the generator is counted up or counted down in accordance with a rotational direction of the zooming cam ring, to thereby find a rotational position of the cam ring, and a focal length and an open F-value are found on the basis of the rotational position.
However, the prior art has the following problem. That is, pulses generated by the pulse generator are counted when a main switch is turned on to angularly move the cam ring toward a long focal point from a condition in which a lens is angularly moved toward a shortest focal point and is stopped. Accordingly, if zooming operations are repeated how many times under such a condition that the lens is not brought to the shortest focal point, information of the lens slips off or shifts gradually under the influence of backlash of gears or the like.